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let's keep in mind the difference between RLA and Tonnage. despite what every BAS guy on the planet thinks, they are not the same thing nor is there a 1 to 1 correlation...not even at '100%'.

as for a multi-compressor system versus a single compressor system, sure a multi-compressor system is going to have a better turn down ratio than a single compressor system, but there are many other factors to consider as well, such as barrel heat transfer rates...the air cooled compressor barrels (that just about every manufacturer uses) have very poor heat transfer compared to just about everybodys centrifugal evaporators...with this factored in, the turn down ratio may not be nearly as significant as it at first seems.

other things to think about are the 2 unit (or more) arrangement. at first this system seems to favor a 2 unit (6 compressors total, RTAC-250, 50 Hz) system, however, this brings up how the unit is piped.

piped in parallel, the maximum flow rate is 1,243 GPM per unit (2,486 GPM total). When the load is low, you will be cycling 2 compressors ON and OFF instead of 1 compressor ON and OFF with a sinlge package unit (centrifugal or air cooled screw)...not nearly as much turn-down-ratio as one would think.

with the chillers piped in series, you have the limitation of 1,243 GPM...less cycling but less range of total flow rate...minimum is 339 GPM per chiller.

my response was actually leaning toward a single package, evap condensing machine, with multiple screws that can do the 500 tons. Single evaporater, no series flow or isolation valves to shut one down on low load, just simple, straight forward chiller package. Yes it exists, and yes it can include pumps, vfd's etc all in one platform. total plant kw would be in the .8kw/ton range, and be rigged to the roof in one shot. there are other thingsa ut there than a trane rtac.

my response was actually leaning toward a single package, evap condensing machine, with multiple screws that can do the 500 tons. Single evaporater, no series flow or isolation valves to shut one down on low load, just simple, straight forward chiller package. Yes it exists, and yes it can include pumps, vfd's etc all in one platform. total plant kw would be in the .8kw/ton range, and be rigged to the roof in one shot. there are other thingsa ut there than a trane rtac.

Sometimes you have to go with where you can get the best service and support. In my area, Trane is pretty much it at the industrial level. Even at the commercial level, they are doing most of the support for all makes.

So, if you have a machine rated at 250 tons and is flowing 600 GPM @ 7 degrees ∆T, its real time capacity is 175 tons. This would be ~87% of its nominal capacity. If the entering condenser water temp is below the nominal, say <85ºF ECWT, when this same chiller is producing 175 tons, the % RLA will be lower than it would be if the entering condenser water temp was 85ºF. This has to do with the pressure differential between condenser and cooler, and how much work the compressor must do to overcome that difference.